Separate/convert bones into their own armature
The option you're looking for is CTRL + ALT +P.

Its called separate bones. All you need to do is select the bones you want to put in their own armature and CTRL + ALT +P. 

Now you have another armature system, with any IK's still working. 
From there if you want to have the bone it was originally attached to be the parent of it, all you would have to do is select the new armature, SHIFT + select the old armature, TAB into pose mode and select the bones you wanted it to be parented too.

Any pose data should still be left intact.